The effect of substrate on TiO2 thin films deposited by atomic layer deposition (ALD)

"ALD is a precision growth technique that can deposit either amorphous or polycrystalline thin films on a variety of substrates. The difference in substrate can cause a variation in the ALD process, even it is carried out using the same reactants and deposition conditions [1]. TiO2 thin films were grown using TTIP (Titanium isopropoxide) ALD on silicon wafers, glass slides, and stainless steel plates in order to study the effect of substrates on the growth of TiO2 with 3,000 deposition cycles, at 300oC.The thin films were analyzed using Xray Diffraction (XRD), Raman Spectroscopy, Atomic Force Microscope (AFM) and Spectroscopic Ellipsometer. The XRD analysis indicates that the main diffraction peak of (101) (2_= 25.3) could be indexed to anatase TiO2, regardless the types of substrates. The results show that crystalline TiO2 thin films could be grown easily on a crystal substrate rather than on an amorphous substrate.

The optical spectra of DMAC based molecules for organic light‐emitting diodes: Hybrid‐exchange density functional theory study

Organic light-emitting diodes (OLED) have considerable advantages over the conventional counterpart. Molecular design by simulations is important for the discovery of new material candidate to improve the performance of OLED. Recently, thermally assisted delayed fluorescence OLED based on DMAC (9,9-dimethyl-9,10-dihydroacridine)-related molecules have been found to have superior performance. In this work, a series of first-principles calculations are performed on DMAC-DPS (diphenylsulfone, emission of blue-color light), DMAC-BP (benzophenone, green), DMAC-DCPP (dicyclohexylphosphonium, red), and the newly designed DMAC-BF (enaminone difluoroboron complexes, red) molecules, based on time-dependent density-functional theory, the hybrid-exchange density functional, and the long-range corrected hybrid-exchange density functional. By varying the percentage of Hartree–Fock (HF) exchange in the hybrid-exchange functional, the emission spectra can be over 97% fitted to the experimental results. We found that the fitted proportion of HF will increase as the wavelengths of the molecules decrease (30% for DPS, 20% for BP, and 10% for DCPP). By contrast, the long-range corrected hybrid-exchange density functional can lead to a good estimate on the absorption spectra. In addition, we have also applied our fitting computational procedure to the newly designed molecule. The molecular orbitals involved in the related excited states have also been investigated for these molecules, which show a common charge-transfer characteristic between the acceptor part (DPS/BP/DCPP/BF) and the donor (DMAC)

Simulating the friction between atomic layers by using a two-q model: Analysis of the relative motion and coherence

Studying friction between atomic layers is not only of great interest for the fundamental aspect of the tribology but also important for many applications such as the layer adhesion in wearable technologies and energy saving. The previous theoretical study has used the modified Prandtl-Tomlinson model to describe the motion of the tip above a two-dimensional atomic layer in an atomic force microscopy experiment. Here the degree of freedom for the substrate has been further explicitly included in the simulation, which is significant because the coherence between the sensing and the substrate layers can be explicitly addressed by computing their relative motion. For both layers, graphene has been chosen as an example for the simulations. Based on the simulations reported here, which agree with the previous relevant theoretical and atomic-force-microscopy experimental results, the motions between the sensing sheet and the substrate can be clearly distinguished. The dependence of motion and force on the parameters for the mechanical properties of the individual layers and the interaction potential between the layers has been carefully studied. For the relatively large values of the parameters for the mechanical properties, the relative motions between the sensing sheet and the substrate show that there would be coherence between the layers, which is beneficial for the adhesion between them. However, many other parameter spaces can be studied further in the future. Similar to the simulations of the motions of the atomic layers, the computed force of the atomic-force-microscopy tip can also indicate the stability of the layers. The theoretical work reported can be used to identify explicitly the relative motions between the sensing sheet and the substrate, providing a substantial improvement for the understanding of the friction between atomic layers. Moreover, in principles, the modeling methodology proposed can be generalized to describe any number of layers in the thin-film devices, by adding a q-parameter for each layer

The adhesive energies between P3HT and PVP for organic electronic devices: Hybrid-exchange density-functional-theory studies

Studying the building blocks for organic electronics – molecules – is important for achieving a great performance for organic electronic devices. Poly(3-hexylthiophene) (P3HT) and povidone (PVP) are common molecules chosen for the semiconducting and dielectric layers of organic electronic devices, respectively. Here we have applied the hybrid-exchange density-functional theory, taking into account empirical dispersion forces and basis set superposition erros, to study the adhesive energies and optimal geometries when integrating the two types of molecules. To ease the analysis of the molecular structures, we have simplified the polymer chain structure to the monomer, dimer, and trimer for the P3HT and PVP. By using B3LYP and BLYP functionals in combination with dispersion forces, we have found the optimal inter-molecular vertical distances between P3HT and PVP are approximately 3.6 Å, 6 Å and 5 Å, for monomer, dimer, and trimer, respectively, with the lowest adsorption energy of ~-0.35, -0.15 and -0.45 eV. However, the sliding effect for the molecular combination is relatively small. These computational results can be potentially compared with the relevant experiments on the molecular crystal structure. The molecular orbitals of the P3HT and PVP molecules show that the charge density is mainly on the five-member rings rather than the polymer chains, which further supports our finite-chain approximation. Our calculations, especially the potential curves, could be useful for the optimal design of molecular structures for organic electronic devices

Density-functional-theory simulations of the water and ice adhesion on silicene quantum dots

The absorption of water and ice on silicon is important to understand for many applications and safety concerns for electronic devices as most of them are fabricated using silicon. Meanwhile, recently silicene nanostructures have attracted much attention due to their potential applications in electronic devices such as gas or humidity sensors. However, for the moment, the theoretical study of the interaction between water molecules and silicene nanostructures is still rare although there is already theoretical work on the effect of water molecules on the silicene periodic structure. The specific conditions such as the finite size effect, the edge saturation of the silicene nanostructure, and the distance between the water/ice and the silicene at the initial onset of the contact have not been carefully considered before. Here we have modelled the absorption of a water molecule and a square ice on the silicene nanodot by using hybrid-exchange density-functional theory, complemented by the Van der Waals forces correction. Three different sizes of silicene nanodots have been chosen for simulations, namely 3×3, 4×4, and 5×5, with and without the hydrogen saturation on the edge. Our calculations suggest that the silicene nanodots chosen here are both hydrophilic and ice-philic. The water molecule and the square ice have tilted angles towards the silicene nanodot plane at ~ 70º and ~ 45º, respectively, which could be owing to the zig–zag structure on silicene. The absorption energies are size dependent for unsaturated silicene nanodots, whereas almost size independent for the hydrogen saturated cases. Our work on the single water molecule absorption energy on silicene nanodots is qualitatively in agreement with the previous theoretical and experimental work. However, the ice structure on silicene is yet to be validated by the relevant experiments. Our calculation results not only further complement the current paucity of water-to-silicene-nanostructure contact mechanisms, but also lead to the first study of square-ice contact mechanisms for silicene. Our findings presented here could be useful for the future design of semiconducting devices based on silicene nanostructures, especially in the humid and low-temperature environments

High Dielectric Constants in BaTiO3 Due to Phonon Mode Softening Induced by Lattice Strains: First Principles Calculations

High-dielectric-constant materials attract much attention due to their broad applications in modern electronics. Barium titanate (BTO) is an established material possessing an ultrahigh dielectric constant; however, a complete understanding of the responsible underlying physical mechanism remains elusive. Here a set of density-functional-theory calculations for the static dielectric tensors of barium titanate under strain has been performed. The dielectric constant increases to ≈7300 under strain. The analysis of the computed vibrational modes shows that transverse vibrational mode softening (the appearance of low-frequency modes) is responsible for this significant increase as driven by the relationship between lattice contribution for the static dielectric constant (k) and vibrational frequency (ω), i.e., urn:x-wiley:27511200:media:apxr202300001:apxr202300001-math-0001. The relevant vibrational mode indicates a large counter-displacement of Ti ions and O anions, which greatly enhances electrical dipoles to screen the electric field. The calculations not only interpreted experimental data on the high dielectric constants of BTO, where the lattice deformation due to the strains from the grain nanostructure plays an important role, but also pointed to exploring high-throughput calculations to facilitate the discovery of the advanced dielectric materials. Moreover, the calculations can prove useful for doped BTO, for which local strains fields can be achieved using defect engineering

An Atomistic-Scale Study for Thermal Conductivity and Thermochemical Compatibility in (DyY)Zr2O7 Combining an Experimental Approach with Theoretical Calculation

Ceramic oxides that have high-temperature capabilities can be deposited on the superalloy components in aero engines and diesel engines to advance engine efficiency and reduce fuel consumption. This paper aims to study doping effects of Dy3+ and Y3+on the thermodynamic properties of ZrO2 synthesized via a sol-gel route for a better control of the stoichiometry, combined with molecular dynamics (MD) simulation for the calculation of theoretical properties. The thermal conductivity is investigated by the MD simulation and Clarke’s model. This can improve the understanding of the microstructure and thermodynamic properties of (DyY)Zr2O7 (DYZ) at the atomistic level. The phonon-defect scattering and phonon-phonon scattering processes are investigated via the theoretical calculation, which provides an effective way to study thermal transport properties of ionic oxides. The measured and predicted thermal conductivity of DYZ is lower than that of 4 mol % Y2O3 stabilized ZrO2 (4YSZ). It is discovered that DYZ is thermochemically compatible with Al2O3 at 1300 °C, whereas at 1350 °C DYZ reacts with Al2O3 forming a small amount of new phases

The role of fluence in determining the response of thin molybdenum films to ultrashort laser irradiation; from laser-induced crystallization to ablation via photomechanical ablation and nanostructure formation

The selective processing of Mo at low temperatures is challenging, especially in advanced manufacturing on flexible and heat-sensitive substrate due to its higher melting temperature. The key role of fluence in determining the response of thin Mo films to ultrashort laser irradiation is considered in this study. At low fluences, the electrical properties of Mo are enhanced by a localized laser-induced crystallization mechanism; the electrical mobility of Mo is increased and the contact resistance between Mo-Si interface is reduced. At higher fluences, selective patterning of Mo proceeds without impacting the Si layer and the threshold fluence for ablation increases with the film thickness of Mo. Two fluence dependent ablation mechanisms are observed depending on the Mo film thickness. For thin films of thicknesses 20 nm and 40 nm, selective ablation proceeds only by a photothermal interaction. For 60 nm and 80 nm thick films, selective ablation proceeds by both photomechanical and photothermal interactions at two-separate higher fluence regimes, respectively. Interestingly, between these two ablation regimes, a non-ablative nanostructuring regime occurs. The study provides a concise overview of the process window for implementing the laser-induced modifications to Mo layers with minimal impact to the substrate using single wavelength ultrashort pulse laser

Surface Modification of Hetero-phase Nanoparticles for Low-Cost Solution-Processable High-k Dielectric Polymer Nanocomposites

The surface modification of nanoparticles (NPs) is crucial for fabricating polymer nanocomposites (NCs) with high dielectric permittivity. Here, we systematically studied the effect of surface functionalization of TiO2 and BaTiO3 NPs to enhance the dielectric permittivity of polyvinylidene fluoride (PVDF) NCs by 23 and 74%, respectively, measured at a frequency of 1 kHz. To further increase the dielectric permittivity of PVDF/NPs-based NCs, we developed a new hetero-phase filler-based approach that is cost-effective and easy to implement. At a 1:3 mixing ratio of TiO2:BaTiO3 NPs, the dielectric constant of the ensuing NC is found to be 50.2, which is comparable with the functionalized BaTiO3-based NC. The highest dielectric constant value of 76.1 measured at 1 kHz was achieved using the (3-aminopropyl)triethoxysilane (APTES)-modified hetero-phase-based PVDF composite at a volume concentration of 5%. This work is an important step toward inexpensive and easy-to-process high-k nanocomposite dielectrics

A graphene nanoplatelet-polydopamine molecularly imprinted biosensor for Ultratrace creatinine detection

Accurate and reliable analysis of creatinine is clinically important for the early detection and monitoring of patients with kidney disease. We report a novel graphene nanoplatelet (GNP)/polydopamine (PDA)-molecularly imprinted polymer (MIP) biosensor for the ultra-trace detection of creatinine in a range of body fluids. Dopamine hydrochloride (DA) monomers were polymerized using a simple one-pot method to form a thin PDA-MIP layer on the surface of GNP with high density of creatinine recognition sites. This novel surface-MIP strategy resulted in a record low limit-of-detection (LOD) of 2 × 10^{−2} pg/ml with a wide dynamic detection range between 1 × 10^{−1}-1 × 10^{9} pg/ml. The practical application of this GNP/PDA-MIP biosensor has been tested by measuring creatinine in human serum, urine, and peritoneal dialysis (PD) fluids. The average recovery rate was 93.7–109.2% with relative standard deviation (RSD) below 4.1% compared to measurements made using standard clinical laboratory methods. Our GNP/PDA-MIP biosensor holds high promise for further development as a rapid, accurate, point-of-care diagnostic platform for detecting and monitoring patients with kidney disease